IPC, Nachos

Tom Kelliher, CS42

Sept. 20, 1996

IPC Mechanisms

Basics: send(), receive() primitives.

Design Issues:

1. Link establishment mechanisms:
   1. Direct or indirect naming.

   2. Circuit or no circuit.

2. More than two processes per link (multicasting).

3. Link buffering:
   1. Zero capacity.

   2. Bounded capacity.

   3. Infinite capacity.

4. Variable- or fixed-size messages.

5. Unidirectional or bidirectional links (symmetry).

6. Resolving lost messages.

7. Resolving out-of-order messages.

8. Resolving duplicated messages.

Mailboxes --- An Indirect Communication Mechanism

Resources owned by kernel.

Messages kept in a queue.

Assume:

1. Only allocating process may execute receive.

2. Any process (including ``owner'') may send.

3. Variable-sized messages.

4. Infinite capacity.

Primitives:

1. int AllocateMB(void)

2. int Send(int mb, char* message)

3. int Receive(int mb, char* message)

4. int FreeMB(int mb)

Example: Process Synchronization

Consider:

Process1()
{
   ...
   S1;
   ...
}

Process2()
{
   ...
   S2;
   ...
}

Example: Tape Drive Allocation and Use

The situation:

Tape allocator process:

initialize();
while (1)
{
   Receive(Tamb, message);
   if (message is a request)

      if (there are enough tape drives)

         for each tape drive being allocated
         {
            fork a handler daemon;
            send daemon mb # in message to requesting process;
            update lists;
         }

      else

         send a rejection message;

   else if (message is a return)
   {
      update lists;
      send an ack message;
   }

   else

      ignore illegal messages;
}

Summary of user process actions:

1. Send request to tape allocator.

2. Receive message back giving mailbox(es) to use in communicating with tape drive(s).

3. Start sending/receiving with tape drive daemon(s).

4. Close tape drives.

5. Send message to tape allocator returning tape drive(s).

Nachos

1. On-line info: http://http.cs.berkeley.edu/~tea/nachos/.

2. Root of Nachos source tree: ~kelliher/pub/cs42/nachos-3.4/.

3. Use a recursive copy to make one copy.

4. nachos group: allows you to share your work.

5. Do a recursive chown to make the group nachos for the source tree.

6. Before each editing session be sure to do the following:

   lisp% umask 7
   

7. Places to find documentation; printing and viewing Postscript files.